Mary Bliss

Radiation detector materials: An overview

Due to events of the past two decades, there has been new and increased usage of radiation-detection technologies for applications in homeland security, nonproliferation, and national defense. As a result, there has been renewed realization of the materials limitations of these technologies and greater demand for the development of next-generation radiation-detection materials. This review describes the current state of radiation-detection material science, with particular emphasis on national security needs and the goal of identifying the challenges and opportunities that this area represents for the materials-science community. Radiation-detector materials physics is reviewed, which sets the stage for performance metrics that determine the relative merit of existing and new materials. Semiconductors and scintillators represent the two primary classes of radiation detector materials that are of interest. The state-of-the-art and limitations for each of these materials classes are presented, along with possible avenues of research. Novel materials that could overcome the need for single crystals will also be discussed. Finally, new methods of material discovery and development are put forward, the goal being to provide more predictive guidance and faster screening of candidate materials and thus, ultimately, the faster development of superior radiation-detection materials.

Glass-fiber-based neutron detectors for high- and low-flux environments

Pacific Northwest Laboratory (PNL) has fabricated cerium-activated lithium silicate scintillating fibers via a hot-downdraw process. These fibers typically have a operational transmission length (e-1 length) of greater than 2 meters. This permits the fabrication of devices which were not possible to consider. Scintillating fibers permit conformable devices, large-area devices, and extremely small devices; in addition, as the thermal-neutron sensitive elements in a fast neutron detection system, scintillating fibers can be dispersed within moderator, improving neutron economy, over that possible with commercially available 3He or BF3 proportional counters. These fibers can be used for national-security applications, in medical applications, in the nuclear-power industry, and for personnel protection at experimental facilities. Data are presented for devices based on single fibers and devices made up of ribbons containing many fibers under high-and low-flux conditions.

Scintillators and applications: Cerium-doped materials

Scintillator materials are widely used for the detection of ionizing radiation in a variety of applications including high energy and nuclear physics, astrophysics, geophysical exploration, medical imaging, security inspection, and industry. Research involving the use of photoluminescence, radioluminescence, thermoluminescence, and synchrotron radiation is underway to further our fundamental understanding of the factors affecting scintillator performance and to discover improved materials. The importance of defects and their effects on energy transfer and overall scintillation efficiency are illustrated by considering recent results for cerium-activated crystals and glasses.

Evidence for dislocations or related defects present in CdTe and Cd1-xZnxTe crystals

Thermoelectric Effect Spectroscopy and Thermally Stimulated Current measurements were used to investigate trapping levels in a semi-insulating CdTe and Cd1-xZnxTe crystals from multiple ingots grown by vertical Bridgman with over pressure control and high-pressure Bridgman methods. The crystals from different growth methods have different dislocation densities as well as Zn concentrations. The thermal ionization energies of these levels were extracted using both the variable heating rate and initial rise methods; the trapping cross sections were then calculated using the temperature maximum method. We report here that the shallow levels observed at E1=0.11+/- 0.02 and E2=0.17+/- 0.02 eV are intrinsic and the latter level is most likely related to the dislocation density.

Final Technical Report for the Neutron Detection without Helium-3 Project

This report details the results of the research and development work accomplished for the ‘Neutron Detection without Helium-3’ project conducted during the 2011-2013 fiscal years. The primary focus of the project was to investigate commercially available technologies that might be used in safeguards applications in the relatively near term. Other technologies that are being developed may be more applicable in the future, but were outside the scope of this study.

The physics and structure-property relationships of scintillator materials: effect of thermal history and chemistry on the light output of scintillating glasses

Abstract The scintillation light output of a series of silicate glasses has been determined for a fixed electron energy deposition. The glasses are cerium-doped lithium-aluminosilicates. Scintillation light output measurements were made as a function of lithium and alumina concentration and of annealing history. In this system, the cerium outer electron cloud serves as a sensitive probe of the local glass structure. From these measurements, a sensitivity matrix relating the light output to the structure is developed and presented. A conceptual model for the behavior of the light output is also presented.

Dense and optical transparent CdWO4 films by sol-gel processing for scintillation applications

In this paper, we report the first successful fabrication of dense and opticallytransparent cadmium tungstate (CWO) films by sol-gel processing and the study oftheir optical and x-ray scintillation properties. A new sol-gel processing method wasdeveloped using tungstic acid and cadmium nitrate as precursors and hydrogenperoxide as solvent; homogeneous and stable CWO sols were aged at roomtemperature and used for the preparation of CWO films. A rapid sintering process wasinvestigated and found to be necessary to make dense and optically transparentnanocrystalline CWO films. CWO films were uniform, fully dense, and crack-free,with CWO as the only detectable crystalline phase, as determined by x-ray diffraction.The thickness, density, grain size, and crystallinity of CWO films are all found to bestrongly dependent on the sintering conditions and in turn impact the optical and x-rayscintillation properties. Sol-gel-derived dense CWO films demonstrated intensephotoluminescence and x-ray excited optical luminescence intensity. The relationshipsbetween sol-gel processing, nanostructures, and optical and x-ray scintillationproperties are discussed in detail.

Hydrothermal growth and photoluminescence property of textured CdWO4 scintillator films

Cadmium tungstate (CWO) films on glass substrate have been first prepared by hydrothermal method at temperatures ranging from 120 to 180 °C from cadmium nitrite and tungstic acid in hydrogen peroxide solution. Crack free and dense CWO films with textured structure and thickness up to 8 μm were formed at 150 and 180 °C as revealed by x-ray diffraction (XRD) and scanning electron microscopy (SEM) studied. Photoluminescence (PL) measurements revealed that highly textured CWO thick films possess better PL property. The growth mechanism and preferred orientation or textured structure as well as the relations between textured structure and PL property have been discussed.

Semi-insulating CdZnTe with improved structural perfection for radiation detector applications

Semi-insulating Cd1-xZnxTe (x = 0.1) with improved structural perfection has been grown using a gradient freeze technique and active control of the Cd partial pressure in the ampoule, both during crystal growth and cool-down of the ingots. The crystal growth was performed in low temperature gradients to minimize thermal stress and achieve material with low dislocation density. Low growth rates were also used to avoid constitutional super-cooling effects. The gradient-freeze technique allowed the growth of large single crystals extending across the entire cross-section of the ingots. The control of the Cd partial pressure allowed the solidification and cool-down of the ingots close to the stoichiometric composition. As a result, the formation and incorporation of large size (>= 1 micrometers diameter) Te inclusions was avoided during crystallization and ingots with high structural perfection were achieved. The improved structural perfection of the material was found to be associated with large spatial variation in the compensation conditions in the ingots and a resulting spatial variation of the bulk electrical resistivity of the material, ranging from 105 (Omega) cm to 1010 (Omega) cm. Samples cut from the high-resistivity sections of the ingots yield detectors exhibiting good spectral performance and an electron mobility-lifetime product of micrometers (tau) e=1.2x10-3 cm2/V.

Performance of a neutron-sensitive scintillating glass fiber panel for portal, freight, and vehicle monitoring

Neutron-sensitive scintillating glass fiber sensors provide several advantages over 3He and BF3 gas-tubes for plutonium detection and surveillance. Large active areas provide significant improvements in sensitivity versus cost. In addition, the glass sensors offer a wide dynamic counting range, fast response time, and low microphonic susceptibility relative to conventional sensors. We report the results of detection limits for neutron glass panels used for portal, freight and vehicle monitoring.